Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320 (if approved) January 03, 2014
Updates: rfc2460 (if approved)
Intended status: Standards Track
Expires: July 7, 2014
The Subnetwork Encapsulation and Adaptation Layer (SEAL)draft-templin-intarea-seal-68.txt
Abstract
This document specifies a Subnetwork Encapsulation and Adaptation
Layer (SEAL). SEAL operates over virtual topologies configured over
connected IP network routing regions bounded by encapsulating border
nodes. These virtual topologies are manifested by tunnels that may
span multiple IP and/or sub-IP layer forwarding hops, where they may
incur packet duplication, packet reordering, source address spoofing
and traversal of links with diverse Maximum Transmission Units
(MTUs). SEAL addresses these issues through the encapsulation and
messaging mechanisms specified in this document.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 7, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (manifested by
tunnels of one form or another) over an actual network that supports
the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual
topologies have elements that appear as one network layer hop, but
are actually multiple IP or sub-IP layer hops which comprise the
"subnetwork" over which the tunnel operates.
The use of IP encapsulation (also known as "tunneling") has long been
considered as the means for creating such virtual topologies (e.g.,
see [RFC2003][RFC2473]). Tunnels serve a wide variety of purposes,
including mobility, security, routing control, traffic engineering,
multihoming, etc., and will remain an integral part of the
architecture moving forward. However, the encapsulation headers
often include insufficiently provisioned per-packet identification
values. IP encapsulation also allows an attacker to produce
encapsulated packets with spoofed source addresses even if the source
address in the encapsulating header cannot be spoofed. A denial-of-
service vector that is not possible in non-tunneled subnetworks is
therefore presented.
Additionally, the insertion of an outer IP header reduces the
effective Maximum Transmission Unit (MTU) visible to the inner
network layer. When IPv6 is used as the encapsulation protocol,
original sources expect to be informed of the MTU limitation through
IPv6 Path MTU discovery (PMTUD) [RFC1981]. When IPv4 is used, this
reduced MTU can be accommodated through the use of IPv4
fragmentation, but unmitigated in-the-network fragmentation has been
deemed harmful through operational experience and studies conducted
over the course of many years [FRAG][FOLK][RFC4963]. Additionally,
classical IPv4 PMTUD [RFC1191] has known operational issues that are
exacerbated by in-the-network tunnels [RFC2923][RFC4459].
The following subsections present further details on the motivation
and approach for addressing these issues.
1.1. Motivation
Before discussing the approach, it is necessary to first understand
the problems. In both the Internet and private-use networks today,
IP is ubiquitously deployed as the Layer 3 protocol. The primary
functions of IP are to provide for routing, addressing, and a
fragmentation and reassembly capability used to accommodate links
with diverse MTUs. While it is well known that the IP address space
is rapidly becoming depleted, there is also a growing awareness that
other IP protocol limitations have already or may soon become
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problematic.
First, the Internet historically provided no means for discerning
whether the source addresses of IP packets are authentic. This
shortcoming is being addressed more and more through the deployment
of site border router ingress filters [RFC2827], however the use of
encapsulation provides a vector for an attacker to circumvent
filtering for the encapsulated packet even if filtering is correctly
applied to the encapsulation header. Secondly, the IP header does
not include a well-behaved identification value unless the source has
included a fragment header for IPv6 or unless the source permits
fragmentation for IPv4. These limitations preclude an efficient
means for routers to detect duplicate packets and packets that have
been re-ordered within the subnetwork. Additionally, recent studies
have shown that the arrival of fragments at high data rates can cause
denial-of-service (DoS) attacks on performance-sensitive networking
gear, prompting some administrators to configure their equipment to
drop fragments unconditionally [I-D.taylor-v6ops-fragdrop].
For IPv4 encapsulation, when fragmentation is permitted the header
includes a 16-bit Identification field, meaning that at most 2^16
unique packets with the same (source, destination, protocol)-tuple
can be active in the network at the same time [RFC6864]. (When
middleboxes such as Network Address Translators (NATs) re-write the
Identification field to random values, the number of unique packets
is even further reduced.) Due to the escalating deployment of high-
speed links, however, these numbers have become too small by several
orders of magnitude for high data rate packet sources such as tunnel
endpoints [RFC4963].
Furthermore, there are many well-known limitations pertaining to IPv4
fragmentation and reassembly - even to the point that it has been
deemed "harmful" in both classic and modern-day studies (see above).
In particular, IPv4 fragmentation raises issues ranging from minor
annoyances (e.g., in-the-network router fragmentation [RFC1981]) to
the potential for major integrity issues (e.g., mis-association of
the fragments of multiple IP packets during reassembly [RFC4963]).
As a result of these perceived limitations, a fragmentation-avoiding
technique for discovering the MTU of the forward path from a source
to a destination node was devised through the deliberations of the
Path MTU Discovery Working Group (MTUDWG) during the late 1980's
through early 1990's which resulted in the publication of [RFC1191].
In this negative feedback-based method, the source node provides
explicit instructions to routers in the path to discard the packet
and return an ICMP error message if an MTU restriction is
encountered. However, this approach has several serious shortcomings
that lead to an overall "brittleness" [RFC2923].
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In particular, site border routers in the Internet have been known to
discard ICMP error messages coming from the outside world. This is
due in large part to the fact that malicious spoofing of error
messages in the Internet is trivial since there is no way to
authenticate the source of the messages [RFC5927]. Furthermore, when
a source node that requires ICMP error message feedback when a packet
is dropped due to an MTU restriction does not receive the messages, a
path MTU-related black hole occurs. This means that the source will
continue to send packets that are too large and never receive an
indication from the network that they are being discarded. This
behavior has been confirmed through documented studies showing clear
evidence of PMTUD failures for both IPv4 and IPv6 in the Internet
today [TBIT][WAND][SIGCOMM][RIPE].
The issues with both IP fragmentation and this "classical" PMTUD
method are exacerbated further when IP tunneling is used [RFC4459].
For example, a tunnel ingress may be required to forward encapsulated
packets into the subnetwork on behalf of hundreds, thousands, or even
more original sources. If the ITE allows IP fragmentation on the
encapsulated packets, persistent fragmentation could lead to
undetected data corruption due to Identification field wrapping
and/or reassembly congestion at the tunnel egress. If the ingress
instead uses classical IP PMTUD it must rely on ICMP error messages
coming from the subnetwork that may be suspect, subject to loss due
to filtering middleboxes, or insufficiently provisioned for
translation into error messages to be returned to the original
sources.
Although recent works have led to the development of a positive
feedback-based end-to-end MTU determination scheme [RFC4821], they do
not excuse tunnels from accounting for the encapsulation overhead
they add to packets. Moreover, in current practice existing
tunneling protocols mask the MTU issues by selecting a "lowest common
denominator" MTU that may be much smaller than necessary for most
paths and difficult to change at a later date. Therefore, a new
approach to accommodate tunnels over links with diverse MTUs is
necessary.
1.2. Approach
This document concerns subnetworks manifested through a virtual
topology configured over a connected network routing region and
bounded by encapsulating border nodes. Example connected network
routing regions include Mobile Ad hoc Networks (MANETs), enterprise
networks, aviation networks and the global public Internet itself.
Subnetwork border nodes forward unicast and multicast packets over
the virtual topology across multiple IP and/or sub-IP layer
forwarding hops that may introduce packet duplication and/or traverse
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links with diverse Maximum Transmission Units (MTUs).
This document introduces a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for tunneling inner network layer protocol packets over
IP subnetworks that connect Ingress and Egress Tunnel Endpoints
(ITEs/ETEs) of border nodes. It provides a modular specification
designed to be tailored to specific associated tunneling protocols.
(A transport-mode of operation is also possible but out of scope for
this document.)
SEAL treats tunnels that traverse the subnetwork as ordinary links
that must support network layer services. Moreover, SEAL provides
dynamic mechanisms (including limited fragmentation and reassembly)
to ensure a maximal path MTU over the tunnel. This is in contrast to
static approaches which avoid MTU issues by selecting a lowest common
denominator MTU value that may be overly conservative for the vast
majority of tunnel paths and difficult to change even when larger
MTUs become available.
1.3. Differences with RFC5320
This specification of SEAL is descended from an experimental
independent RFC publication of the same name [RFC5320]. However,
this specification introduces a number of fundamental differences
from the earlier publication. This specification therefore obsoletes
(i.e., and does not update) [RFC5320].
First, [RFC5320] forms a 32-bit Identification value by concatenating
the 16-bit IPv4 Identification field with a 16-bit Identification
"extension" field in the SEAL header. This means that [RFC5320] can
only operate over IPv4 networks (since IPv6 headers do not include a
16-bit version number) and that the SEAL Identification value can be
corrupted if the Identification in the outer IPv4 header is
rewritten. In contrast, this specification includes a 32-bit
Identification value that is independent of any identification fields
found in the inner or outer IP headers, and is therefore compatible
with any inner and outer IP protocol version combinations.
Additionally, the SEAL fragmentation and reassembly procedures
defined in [RFC5320] differ significantly from those found in this
specification. In particular, this specification defines an 13-bit
Offset field that allows for finer-grained fragment sizes when SEAL
fragmentation and reassembly is necessary. In contrast, [RFC5320]
includes only a 3-bit Segment field and performs reassembly through
concatenation of consecutive segments.
Finally, SEAL no longer uses the IPv4 fragmentation sensing method
specified in [RFC5320] as well as in earlier versions of this
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document. This departure is based on the fact that there is no way
for the ITE or ETE to control the way in which middleboxes perform
IPv4 fragmentation (e.g., largest fragment first, smallest fragment
first, all fragments the same size, etc.). Moreover, there may be
middleboxes in the path that reassemble IPv4 fragmented packets
before delivering them to the ETE as the final destination. Use of
IPv4 fragmentation sensing in the ETE also greatly complicated the
specification and proved difficult to implement. Therefore, although
the IPv4 fragmentation sensing method is conceptually elegant and
natural, it is no longer included.
2. Terminology
The following terms are defined within the scope of this document:
subnetwork
a virtual topology configured over a connected network routing
region and bounded by encapsulating border nodes.
IP
used to generically refer to either Internet Protocol (IP)
version, i.e., IPv4 or IPv6.
Ingress Tunnel Endpoint (ITE)
a portal over which an encapsulating border node (host or router)
sends encapsulated packets into the subnetwork.
Egress Tunnel Endpoint (ETE)
a portal over which an encapsulating border node (host or router)
receives encapsulated packets from the subnetwork.
inner packet
an unencapsulated network layer protocol packet (e.g., IPv4
[RFC0791], OSI/CLNP [RFC0994], IPv6 [RFC2460], etc.) before any
outer encapsulations are added. Internet protocol numbers that
identify inner packets are found in the IANA Internet Protocol
registry [RFC3232]. SEAL protocol packets that incur an
additional layer of SEAL encapsulation are also considered inner
packets.
outer IP packet
a packet resulting from adding an outer IP header (and possibly
other outer headers) to a SEAL-encapsulated inner packet.
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packet-in-error
the leading portion of an invoking data packet encapsulated in the
body of an error control message (e.g., an ICMPv4 [RFC0792] error
message, an ICMPv6 [RFC4443] error message, etc.).
Packet Too Big (PTB) message
a control plane message indicating an MTU restriction (e.g., an
ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
"Fragmentation Needed" message [RFC0792], etc.).
Don't Fragment (DF) bit
a bit that indicates whether the packet may be fragmented by the
network. The DF bit is explicitly included in the IPv4 header
[RFC0791] and may be set to '0' to allow fragmentation or '1' to
disallow further in-network fragmentation. The bit is absent from
the IPv6 header [RFC2460], but implicitly set to '1' because
fragmentation can occur only at IPv6 sources.
3. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. When used
in lower case (e.g., must, must not, etc.), these words MUST NOT be
interpreted as described in [RFC2119], but are rather interpreted as
they would be in common English.
4. Applicability Statement
SEAL was originally motivated by the specific case of subnetwork
abstraction for Mobile Ad hoc Networks (MANETs), however the domain
of applicability also extends to subnetwork abstractions over
enterprise networks, mobile networks, aviation networks, ISP
networks, SO/HO networks, the global public Internet itself, and any
other connected network routing region.
SEAL provides a network sublayer used during encapsulation of an
inner network layer packet within outer encapsulating headers. SEAL
can also be used as a sublayer within a transport layer protocol data
payload, where transport layer encapsulation is typically used for
Network Address Translator (NAT) traversal as well as operation over
subnetworks that give preferential treatment to certain "core"
Internet protocols, e.g., TCP, UDP, etc. (However, note that TCP
encapsulation may not be appropriate for all use cases; particularly
those that require low delay and/or delay variance.) The SEAL header
is processed in the same manner as for IPv6 extension headers, i.e.,
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it is not part of the outer IP header but rather allows for the
creation of an arbitrarily extensible chain of headers in the same
way that IPv6 does.
To accommodate MTU diversity, the Ingress Tunnel Endpoint (ITE) may
need to perform limited fragmentation which the Egress Tunnel
Endpoint (ETE) reassembles. The ITE and ETE further engage in
minimal path probing to determine when the path can be traversed
without fragmentation. This allows the ITE to send whole packets
instead of fragmented packets whenever possible.
In practice, SEAL is typically used as an encapsulation sublayer in
conjunction with existing tunnel types such as IPsec [RFC4301] ,
GRE[RFC1701], IP-in-IPv6 [RFC2473], IP-in-IPv4 [RFC4213][RFC2003],
etc. When used with existing tunnel types that insert mid-layer
headers between the inner and outer IP headers (e.g., IPsec, GRE,
etc.), the SEAL header is inserted between the mid-layer headers and
outer IP header.
5. SEAL Specification
The following sections specify the operation of SEAL:
5.1. SEAL Tunnel Model
SEAL is an encapsulation sublayer used within point-to-point, point-
to-multipoint, and non-broadcast, multiple access (NBMA) tunnels.
SEAL can also be used with multicast-capable tunnels, but the path
probing mechanisms specified in the following sections may not always
be sufficient to determine an optimal MTU for a multicast group.
Each tunnel is configured over one or more underlying interfaces
attached to subnetwork links, where each link represents a different
subnetwork path. The tunnel connects an ITE to one or more ETE
"neighbors" via encapsulation across an underlying subnetwork, where
each tunnel neighbor relationship is maintained over one or more
subnetwork paths. The tunnel neighbor relationship may be
bidirectional, partially unidirectional or fully unidirectional.
A bidirectional tunnel neighbor relationship is one over which both
tunnel endpoints can exchange both data and control messages. A
partially unidirectional tunnel neighbor relationship allows the near
end ITE to send data packets forward to the far end ETE, while the
far end only returns control messages when necessary. Finally, a
fully unidirectional mode of operation is one in which the near end
ITE can receive neither data nor control messages from the far end
ETE.
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SEAL-enabled ITEs encapsulate each inner packet in any ancillary
tunnel protocol headers and trailers, a SEAL header, and any outer
header encapsulations as shown in Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL header ~
+--------------------+
~ tunnel headers ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
~ tunnel trailers ~
+--------------------+
Figure 1: SEAL Encapsulation
The ITE inserts the SEAL header according to the specific tunneling
protocol. For simple encapsulation of an inner network layer packet
within an outer IP header, the ITE inserts the SEAL header following
the outer IP header and before the inner packet as: IP/SEAL/{inner
packet}.
For encapsulations over transports such as UDP, the ITE inserts the
SEAL header following the outer transport layer header and before the
inner packet, e.g., as IP/UDP/SEAL/{inner packet}. In that case, the
UDP header is seen as an "other outer header" as depicted in Figure 1
and the outer IP and transport layer headers are together seen as the
outer encapsulation headers. (Note that outer transport layer
headers such as UDP must sometimes be included to ensure that SEAL
packets will traverse the path to the ETE without loss due filtering
middleboxes. The ETE MUST accept both IP/SEAL and IP/UDP/SEAL as
equivalent packets so that the ITE can discontinue outer transport
layer encapsulation if the path supports raw IP/SEAL encapsulation.)
For SEAL encapsulations that involve tunnel types that include
ancillary tunnel headers (e.g., GRE, IPsec, etc.) the ITE inserts the
SEAL header as a leading extension to the tunnel headers, i.e., the
SEAL encapsulation appears as part of the same tunnel and not a
separate tunnel. For example, for GRE the ITE inserts the SEAL
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header as IP/SEAL/GRE/{inner packet}, and for IPsec the ITE inserts
the SEAL header as IP/SEAL/IPsec-header/{inner packet}/IPsec-trailer.
In such cases, SEAL considers the length of the inner packet only
(i.e., and not the other tunnel headers and trailers) when performing
its packet size calculations.
SEAL supports both "nested" tunneling and "re-encapsulating"
tunneling. Nested tunneling occurs when a first tunnel is
encapsulated within a second tunnel, which may then further be
encapsulated within additional tunnels. Nested tunneling can be
useful, and stands in contrast to "recursive" tunneling which is an
anomalous condition incurred due to misconfiguration or a routing
loop. Considerations for nested tunneling and avoiding recursive
tunneling are discussed in Section 4 of [RFC2473] as well as in
Section 9 of this document.
Re-encapsulating tunneling occurs when a packet arrives at a first
ETE, which then acts as an ITE to re-encapsulate and forward the
packet to a second ETE connected to the same subnetwork. In that
case each ITE/ETE transition represents a segment of a bridged path
between the ITE nearest the source and the ETE nearest the
destination. Uses for re-encapsulating tunneling are discussed in
[I-D.templin-aerolink]. Combinations of nested and re-encapsulating
tunneling are also naturally supported by SEAL.
The SEAL ITE considers each underlying interface as the ingress
attachment point to a separate subnetwork path to the ETE. The ITE
therefore may experience different path MTUs on different subnetwork
paths.
Finally, the SEAL ITE ensures that the inner network layer protocol
will see a minimum MTU of 1500 bytes over each subnetwork path
regardless of the outer network layer protocol version, i.e., even if
a small amount of fragmentation and reassembly are necessary. This
is to avoid path MTU "black holes" for the minimum MTU configured by
the vast majority of links in the Internet.
5.3. SEAL Encapsulation Format
The SEAL header shares the same format and IP protocol number ('44')
as the IPv6 Fragment Header specified in Section 4.5 of [RFC2460].
The SEAL header is differentiated from the IPv6 Fragment Header by
defining bit number 30 as the "SEAL (S)" bit which is set to 1 when
SEAL encapsulation is used and set to 0 for ordinary IPv6
fragmentation. SEAL therefore updates the IPv6 Fragment Header
specification as shown in Figure 2:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Reserved | Fragment Offset |R|S|M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SEAL Encapsulation Format
5.4. ITE Specification5.4.1. Tunnel MTU
The tunnel must present a stable MTU value to the inner network layer
as the size for admission of inner packets into the tunnel. Since
tunnels may support a large set of subnetwork paths that accept
widely varying maximum packet sizes, however, a number of factors
should be taken into consideration when selecting a tunnel MTU.
Due to the ubiquitous deployment of standard Ethernet and similar
networking gear, the nominal Internet cell size has become 1500
bytes; this is the de facto size that end systems have come to expect
will either be delivered by the network without loss due to an MTU
restriction on the path or a suitable ICMP Packet Too Big (PTB)
message returned. When large packets sent by end systems incur
additional encapsulation at an ITE, however, they may be dropped
silently within the tunnel since the network may not always deliver
the necessary PTBs [RFC2923]. The ITE SHOULD therefore set a tunnel
MTU of at least 1500 bytes and provide accommodations to ensure that
packets up to that size are successfully conveyed to the ETE.
The inner network layer protocol consults the tunnel MTU when
admitting a packet into the tunnel. For non-SEAL inner IPv4 packets
with the IPv4 Don't Fragment (DF) bit cleared (i.e., DF==0), if the
packet is larger than the tunnel MTU the inner IPv4 layer uses IPv4
fragmentation to break the packet into fragments no larger than the
MTU. The ITE then admits each fragment into the tunnel as an
independent packet.
For all other inner packets, the inner network layer admits the
packet if it is no larger than the tunnel MTU; otherwise, it drops
the packet and sends a PTB error message to the source with the MTU
value set to the MTU. The message contains as much of the invoking
packet as possible without the entire message exceeding the network
layer minimum MTU size.
The ITE can alternatively set an indefinite tunnel MTU such that all
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inner packets are admitted into the tunnel regardless of their size
(practical maximums are 64KB for IPv4 and 4GB for IPv6 [RFC2675]).
For ITEs that host applications that use the tunnel directly, this
option must be carefully coordinated with protocol stack upper layers
since some upper layer protocols (e.g., TCP) derive their packet
sizing parameters from the MTU of the outgoing interface and as such
may select too large an initial size. This is not a problem for
upper layers that use conservative initial maximum segment size
estimates and/or when the tunnel can reduce the upper layer's maximum
segment size, e.g., by reducing the size advertised in the MSS option
of outgoing TCP messages (sometimes known as "MSS clamping").
In light of the above considerations, the ITE SHOULD configure an
indefinite MTU on *router* tunnels so that SEAL performs all
subnetwork adaptation from within the tunnel as specified in the
following sections. The ITE MAY instead set a smaller MTU on *host*
tunnels; in that case, the RECOMMENDED MTU is the maximum of 1500
bytes and the smallest MTU among all of the underlying links minus
the size of the encapsulation headers.
5.4.2. Tunnel Neighbor Soft State
The ITE maintains a number of soft state variables and constants.
The ITE maintains a per-ETE window of Identification values for the
packets it sends to the ETE. The ITE increments the current
Identification value monotonically (modulo 2^32) for each packet it
sends.
For each subnetwork path, the ITE must also account for encapsulation
header lengths. The ITE therefore maintains the per subnetwork path
constant values "SHLEN" set to the length of the SEAL header, "THLEN"
set to the length of the outer encapsulating transport layer headers
(or 0 if outer transport layer encapsulation is not used), "IHLEN"
set to the length of the outer IP layer header, and "HLEN" set to
(SHLEN+THLEN+IHLEN). When calculating these lengths, the ITE must
include the length of the uncompressed headers even if header
compression is enabled. When SEAL is used in conjunction with tunnel
types that insert additional headers/trailers such as GRE or IPsec,
the length of the additional headers and trailers is also included in
the HLEN calculation.
The ITE also sets a global constant value "MINMTU" to 1500 bytes and
sets a per subnetwork path constant value 'FRAGMTU' to (1280-HLEN)
bytes (where 1280 is the minimum path MTU for IPv6 [RFC2460]). The
value 1280 is used regardless of the outer IP protocol version even
though the practical minimum MTU for IPv4 is only 576 bytes [RFC1122]
and the theoretical minimum MTU for IPv4 is only 68 bytes [RFC0791].
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The value 1280 is applied also to IPv4 since IPv4 links with MTUs
smaller than 1280 are presumably performance-constrained such that
IPv4 fragmentation can be used to accommodate MTU underruns without
risk of high data rate reassembly misassociations.
The ITE also sets a per subnetwork path variable "MAXMTU" to the
maximum of MINMTU and the MTU of the underlying interface minus HLEN.
The ITE thereafter adjusts MAXMTU based on any PTB messages it
receives from the subnetwork, but does not reduce MAXMTU below
MINMTU.
The ITE finally maintains a per subnetwork path boolean variable
"DOFRAG", which is initially set to TRUE and may be reset to FALSE if
the ITE discovers that the MTU on the path to the ETE is sufficient
to accommodate packet sizes of MINMTU bytes or larger.
5.4.3. SEAL Layer Pre-Processing
The SEAL layer is logically positioned between the inner and outer
network protocol layers, where the inner layer is seen as the (true)
network layer and the outer layer is seen as the (virtual) data link
layer. Each packet to be processed by the SEAL layer is either
admitted into the tunnel by the inner network layer protocol as
described in Section 5.4.1 or is undergoing re-encapsulation from
within the tunnel. The SEAL layer sees the former class of packets
as inner packets that include inner network and transport layer
headers, and sees the latter class of packets as transitional SEAL
packets that include the outer and SEAL layer headers that were
inserted by the previous hop SEAL ITE. For these transitional
packets, the SEAL layer re-encapsulates the packet with new outer and
SEAL layer headers when it forwards the packet to the next hop SEAL
ITE.
We now discuss the SEAL layer pre-processing actions for these two
classes of packets.
5.4.3.1. Inner Packet Pre-Processing
For each IPv4 inner packet with DF==0 in the IP header, if the packet
is larger than MINMTU bytes the ITE first uses standard IPv4
fragmentation to fragment the packet into N pieces of at most MINMTU
bytes each. In this process, the ITE MUST additionally ensure that N
is minimized, the first fragment is the largest fragment and no
fragments are overlapping. The ITE then submits each fragment for
SEAL encapsulation as specified in Section 5.4.4.
For all other inner packets, if the packet is no larger than MAXMTU
the ITE submits it for SEAL encapsulation as specified in Section
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5.4.4. Otherwise, the ITE discards the packet and sends a PTB
message appropriate to the inner protocol version (subject to rate
limiting) with the MTU field set to MAXMTU.
5.4.3.2. Transitional SEAL Packet Pre-Processing
For each transitional packet that is to be processed by the SEAL
layer from within the tunnel, if the packet is larger than MAXMTU for
the next hop subnetwork path the ITE discards the packet and sends a
PTB message appropriate to the inner protocol version (subject to
rate limiting) with the MTU field set to MAXMTU. Otherwise, the ITE
sets aside the encapsulating SEAL and outer headers for later
reference (see Section 5.4.5) and submits the inner packet for SEAL
re-encapsulation as discussed in the following sections.
5.4.4. SEAL Encapsulation and Fragmentation
For each inner packet/fragment submitted for SEAL encapsulation, the
ITE next encapsulates the packet in a SEAL header formatted as
specified in Section 5.3. The ITE next sets S=1 and sets the Next
Header field to the protocol number corresponding to the address
family of the encapsulated inner packet. For example, the ITE sets
the Next Header field to the value '4' for encapsulated IPv4 packets
[RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213],
'47' for GRE [RFC1701], '80' for encapsulated OSI/CLNP packets
[RFC1070], etc.
Next, if the inner packet is no larger than FRAGMTU, or if the inner
packet is larger than MINMTU, or if the DOFRAG flag is FALSE, the ITE
sets (M=0; Offset=0) and considers the packet an "atomic fragment"
(see: [RFC6946]). Otherwise, the ITE fragments the inner packet
using the fragmentation procedures specified in Section 4.5 of
[RFC2460]. In this process, the ITE breaks the inner packet into two
non-overlapping fragments, where the encapsulated SEAL packet
containing the first fragment MUST be as large as possible without
exceeding 1280 bytes (i.e., the IPv6 minimum MTU) and the
encapsulated SEAL packet containing the second fragment MUST include
the remainder of the inner packet. This ensures that the entire IP
header (plus extensions) is likely to fit within the first fragment
and that the number of fragments is minimized. The ITE then adds the
outer encapsulating headers as specified in Section 5.4.5.
5.4.5. Outer Encapsulation
Following SEAL encapsulation and fragmentation, the ITE next
encapsulates each fragment in the requisite outer transport (when
necessary) and IP layer headers. When a transport layer header such
as UDP or TCP is included, the ITE writes the port number for SEAL in
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the transport destination service port field.
When UDP encapsulation is used, the ITE sets the UDP checksum field
to zero for both IPv4 and IPv6 packets (see: [RFC6935][RFC6936]).
The ITE then sets the outer IP layer headers the same as specified
for ordinary IP encapsulation (e.g., [RFC1070][RFC2003], [RFC2473],
[RFC4213], etc.) except that for ordinary SEAL packets the ITE copies
the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion
Experienced" values in the inner network layer header into the
corresponding fields in the outer IP header. For transitional SEAL
packets undergoing re-encapsulation, the ITE instead copies the "TTL/
Hop Limit", "Type of Service/Traffic Class" and "Congestion
Experienced" values in the original outer IP header of the
transitional packet into the corresponding fields in the new outer IP
header of the packet to be forwarded (i.e., the values are
transferred between outer headers and *not* copied from the inner
network layer header).
The ITE also sets the IP protocol number to the appropriate value for
the first protocol layer within the encapsulation (e.g., UDP, TCP,
IPv6 Fragment Header, etc.). When IPv6 is used as the outer IP
protocol, the ITE then sets the flow label value in the outer IPv6
header the same as described in [RFC6438]. When IPv4 is used as the
outer IP protocol, if the encapsulated SEAL packet is no larger than
1280 bytes the ITE sets DF=0 in the IPv4 header to allow the packet
to be fragmented if it encounters a restricting link; otherwise, the
ITE sets DF=1 (for IPv6 subnetwork paths, the DF bit is absent but
implicitly set to 1). The ITE finally sends each outer packet via
the corresponding underlying subnetwork path.
5.4.6. Path MTU Probing and ETE Reachability Verification
When the ITE is actively sending packets over a subnetwork path to an
ETE, it also sends explicit probes subject to rate limiting to test
the path MTU. To generate a probe, the ITE creates an ICMPv6 Echo
Request message [RFC4443] of length MINMTU bytes and encapsulates the
message in a SEAL header and any other outer headers, i.e., with the
length of the resulting SEAL packet being (MINMTU+HLEN) bytes. It
then sets (Offset=0; S=1; M=0) in the SEAL header, and also sets DF=1
in the outer IP header when IPv4 is used. It finally writes the
value '58' in the Next Header field of the SEAL header to indicate
that the message is a SEAL-encapsulated ICMPv6 message.
The ITE sends such MINMTU probes to determine whether SEAL
fragmentation is still necessary (see Section 5.4.4). In particular,
if the ITE sends a probe and receives a SEAL-encapsulated ICMPv6 Echo
Reply message probe reply (see: section 5.5.4), it SHOULD set DOFRAG
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for this subnetwork path to FALSE. Note that the nominal probe size
of MINMTU bytes is RECOMMENDED since probes slightly smaller than
this size may be fragmented by the ITE of a nested tunnel further
down the path. For example, a successful probe size of 1400 bytes
does not guarantee that fragmentation is not occurring at the ITE of
another tunnel nesting level. While this would not necessarily
result in communication failure, it could yield poor performance not
only for the other tunnel nesting levels but also for the ITE itself.
The ITE can also send smaller probes to determine whether the ETE is
still reachable over this subnetwork path. The ITE prepares the
probe as described above then sends the message to the ETE. If the
ITE receives a probe reply, its upper layers can consider the message
as a reachability indication. The ITE can also send larger probes to
test for larger MTU sizes; however, SEAL considers probing for MTU
sizes larger than MINMTU as an end-to-end consideration to be
addressed by end systems (see: Section 7).
Finally, the ITE can also send probes to detect whether an outer
transport layer header is no longer necessary to reach this ETE. For
example, if the ITE sends its initial packets as IP/UDP/SEAL/*, it
can send probes constructed as IP/SEAL/[probe] to determine whether
the ETE is reachable without the use of UDP encapsulation. If so,
the ITE should also send a new MINMTU probe since switching to a new
encapsulation format may result in a path change.
While probing, the ITE processes ICMP messages as specified in
Section 5.4.7.
5.4.7. Processing ICMP Messages
When the ITE sends SEAL packets, it may receive ICMP error messages
[RFC0792][RFC4443] from a router on the path to the ETE. Each ICMP
message includes an outer IP header, followed by an ICMP header,
followed by a portion of the SEAL packet that generated the error
(also known as the "packet-in-error"). Note that the ITE may receive
an ICMP message from either an ordinary router on the path or from
another ITE that is at the head end of a nested level of
encapsulation. The ITE has no security associations with this nested
ITE, hence it should consider the message the same as if it
originated from an ordinary router.
The ITE should process ICMP Protocol/Port Unreachable messages as a
hint that the ETE does not implement SEAL. The ITE can optionally
ignore other ICMP messages that do not include sufficient information
in the packet-in-error, or process them as a hint that the subnetwork
path to the ETE may be failing. The ITE then discards these types of
messages.
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For other ICMP messages, the ITE SHOULD examine the SEAL data packet
within the packet-in-error field. If the IP source and/or
destination addresses are invalid, or if the value in the SEAL header
Identification field (if present) is not within the window of packets
the ITE has recently sent to this ETE, the ITE discards the message.
Next, if the received ICMP message is a PTB the ITE sets MAXMTU to
the maximum of MINMTU and the MTU value in the message minus HLEN.
If the MTU value in the message is smaller than (MINMTU+HLEN), the
ITE also resets DOFRAG to TRUE and discards the message.
If the ICMP message was not discarded, the ITE transcribes it into a
message appropriate for the inner protocol version (e.g., ICMPv4 for
IPv4, ICMPv6 for IPv6, etc.) and forwards the transcribed message to
the previous hop toward the inner source address.
5.4.8. Detecting Path MTU Changes
The ITE SHOULD periodically reset MAXMTU to the MTU of the underlying
subnetwork interface to determine whether the subnetwork path MTU has
increased. If the path still has a too-small MTU, the ITE will
receive a PTB message that reports a smaller size.
5.5. ETE Specification5.5.1. Reassembly Buffer Requirements
The ETE MUST configure a minimum SEAL reassembly buffer size of
(MINMTU+HLEN) bytes for the reassembly of fragmented SEAL packets
(see: Section 5.5.4). Note that the value "HLEN" may be variable and
initially unknown to the ETE. It is therefore RECOMMENDED that the
ETE configure a slightly larger SEAL reassembly buffer size of 2048
bytes (2KB).
When IPv4 is used as the outer layer of encapsulation, the ETE MUST
also configure a minimum IPv4 reassembly buffer size of 1280 bytes.
5.5.2. Tunnel Neighbor Soft State
The ETE maintains a window of Identification values for the packets
it has recently received from this ITE as well as a window of
Identification values for the packets it has recently sent to this
ITE.
5.5.3. IPv4-Layer Reassembly
The ETE reassembles fragmented IPv4 packets that are explicitly
addressed to itself. For IPv4 fragments of SEAL packets, the ETE
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SHOULD maintain conservative reassembly cache high- and low-water
marks. When the size of the reassembly cache exceeds this high-water
mark, the ETE SHOULD actively discard stale incomplete reassemblies
(e.g., using an Active Queue Management (AQM) strategy) until the
size falls below the low-water mark. The ETE SHOULD also actively
discard any pending reassemblies that clearly have no opportunity for
completion, e.g., when a considerable number of new fragments have
arrived before a fragment that completes a pending reassembly
arrives.
The ETE processes IPv4 fragments as specified in the normative
references, i.e., it performs any necessary IPv4 reassembly then
submits the packet to the appropriate upper layer protocol module.
For SEAL packets, the ETE then performs SEAL decapsulation as
specified in Section 5.5.4.
5.5.4. Decapsulation, SEAL-Layer Reassembly, and Re-Encapsulation
For each SEAL packet accepted for decapsulation, the ETE first
examines the Identification field. If the Identification is not
within the window of acceptable values for this ITE, the ETE silently
discards the packet..
Next, if the SEAL header has (Offset!=0 || M=1) the ETE submits the
packet for reassembly as specified for IPv6 reassembly in Section 4.5
of [RFC2460]. During the reassembly process, the ETE discards any
fragments that are overlapping with respect to fragments that have
already been received (see: [RFC5722]), and also discards any
fragments that have M=1 in the SEAL header but do not contain an
integer multiple of 8 bytes. The ETE further SHOULD manage the SEAL
reassembly cache the same as described for the IPv4-Layer Reassembly
cache in Section 5.5.3, i.e., it SHOULD perform an early discard for
any pending reassemblies that have low probability of completion.
Next, if the (reassembled) packet is an ICMPv6 Echo Request probe
message, the ETE prepares an ICMPv6 Echo Reply probe reply message to
send back to the ITE. The ETE then encapsulates the probe reply as
specified in Section 5.4.4 and fragments the message if necessary
according to the DOFRAG flag (i.e., to ensure that the probe reply is
delivered to the ITE). The ETE then sends the probe reply to the ITE
and discards the probe. When the ITE receives the probe reply, it
reassembles the message if necessary and processes it as specified in
Section 5.4.6.
Finally, the ETE discards the outer headers of the (reassembled)
packet and processes the inner packet according to the header type
indicated in the SEAL Next Header field. If the next hop toward the
inner destination address is via a different interface than the SEAL
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packet arrived on, the ETE discards the SEAL and outer headers and
delivers the inner packet either to the local host or to the next hop
if the packet is not destined to the local host.
If the next hop is on the same tunnel the SEAL packet arrived on,
however, the ETE submits the packet for SEAL re-encapsulation
beginning with the specification in Section 5.4.3 above and without
decrementing the value in the inner (TTL / Hop Limit) field.
6. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
7. End System Requirements
End systems are encouraged to implement end-to-end MTU assurance
(e.g., using Packetization Layer Path MTU Discovery (PLPMTUD) per
[RFC4821]) even if the subnetwork is using SEAL.
When end systems use PLPMTUD, SEAL will ensure that the tunnel
behaves as a link in the path that assures an MTU of at least 1500
bytes while still allowing end systems to discover larger MTUs. The
PLPMTUD mechanism will therefore be able to function as designed in
order to discover and utilize larger MTUs.
8. Router Requirements
Routers within the subnetwork are expected to observe the standard IP
router requirements, including the implementation of IP fragmentation
and reassembly as well as the generation of ICMP messages
[RFC0792][RFC1122][RFC1812][RFC2460][RFC4443][RFC6434].
Note that, even when routers support existing requirements for the
generation of ICMP messages, these messages are often filtered and
discarded by middleboxes on the path to the original source of the
message that triggered the ICMP. It is therefore not possible to
assume delivery of ICMP messages even when routers are correctly
implemented.
9. Multicast/Anycast Considerations
On multicast-capable tunnels, encapsulated packets sent by an ITE may
be received by potentially many ETEs. In that case, the ITE can
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still send unicast probe messages to receive probe replies from a
specific ETE, or it can send multicast probe messages to receive
replies from all ETEs in the multicast group that receive the probe.
If the ITE were to send a multicast MINMTU probe message as described
in Section 5.4.6, however, it would be unable to discern whether all
ETEs received the probe unless it had some way of tracking the full
constituency of the multicast group. For multicast ETE addresses,
the ITE would therefore ordinarily set MAXMTU=MINMTU and DOFRAG=TRUE.
But, the setting of these values may be situation-dependent and based
on whether the ITE can tolerate packet loss to ETEs that may be
reached by subnetwork paths having small MTUs.
For ETEs that configure an anycast address, if the ITE sends a MINMTU
probe message it may receive a probe reply from a first ETE but then
be re-routed to a second ETE. It is therefore necessary for the ITE
to continue to send periodic probes (subject to rate limiting) as
described in Section 5.4.6 so that any path oscillations between ETEs
that configure the same anycast address will not result in a
sustained path MTU black hole.
10. Compatibility Considerations
Since SEAL is based on the standard IPv6 fragment header, the ITE can
implement the scheme independently of any ETE implementations.
Therefore, if the ITE uses SEAL but the ETE does not the ITE can
still send a MINMTU probe as specified in Section 5.4.6 but may
receive an ordinary (i.e., non SEAL-encapsulated) probe reply. If
so, it SHOULD reset DOFRAG to FALSE the same as if the ETE returned a
SEAL-encapsulated probe reply.
In some cases, a non-SEAL ETE may not be able to reassemble
fragmented SEAL packets up to (MINMTU+HLEN) bytes, since [RFC2460]
only requires IPv6 nodes to reassemble packets up to 1500 bytes in
length. To test for this condition, the ITE can create a MINMTU
probe message, fragment the message into two pieces, then send both
fragments to the ETE. If the ETE returns a probe reply, the ITE has
assurance that the ETE is capable of reassembly. Otherwise, the ITE
SHOULD reset MAXMTU for this subnetwork path to (MINMTU-HLEN) or even
smaller if the ETE still cannot accept packets of this size.
11. Nested Encapsulation Considerations
SEAL supports nested tunneling - an example would be a recursive
nesting of mobile networks, where the first network receives service
from an ISP, the second network receives service from the first
network, the third network receives service from the second network,
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etc. Since it is imperative that such nesting not extend
indefinitely, tunnels that use SEAL SHOULD honor the Encapsulation
Limit option defined in [RFC2473].
12. Reliability Considerations
Although a tunnel may span an arbitrarily-large subnetwork expanse,
the IP layer sees the tunnel as a simple link that supports the IP
service model. Links with high bit error rates (BERs) (e.g., IEEE
802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] to
increase packet delivery ratios, while links with much lower BERs
typically omit such mechanisms. Since Tunnels may traverse
arbitrarily-long paths over links of various types that are already
either performing or omitting ARQ as appropriate, it would therefore
be inefficient to require the tunnel endpoints to also perform ARQ.
13. Integrity Considerations
Fragmentation and reassembly schemes must consider packet-splicing
errors, e.g., when two fragments from the same packet are
concatenated incorrectly, when a fragment from packet X is
reassembled with fragments from packet Y, etc. The primary sources
of such errors include implementation bugs and wrapping ID fields.
In particular, the IPv4 16-bit ID field can wrap with only 64K
packets with the same (src, dst, protocol)-tuple alive in the system
at a given time [RFC4963]. When the IPv4 ID field is re-written by a
middlebox such as a NAT or Firewall, ID field wrapping can occur with
even fewer packets alive in the system.
Fortunately, SEAL includes a 32-bit ID field the same as for IPv6
fragmentation and also only employs SEAL fragmentation for packets up
to 1500 bytes in length. SEAL also only allows IPv4 network
fragmentation for packets up to 1280 bytes in length, but this size
is small enough to fit within the MTU of modern high-speed IPv4 links
without fragmentation. IPv4 links with smaller MTUs certainly exist,
but typically support data rates that are slow enough to preclude
high data rate reassembly misassociations errors; hence, a small
amount of IPv4 fragmentation is deemed acceptable.
14. IANA Considerations
The IANA is requested to allocate a User Port number for "SEAL" in
the 'port-numbers' registry. The Service Name is "SEAL", and the
Transport Protocols are TCP and UDP. The Assignee is the IESG
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(iesg@ietf.org) and the Contact is the IETF Chair (chair@ietf.org).
The Description is "Subnetwork Encapsulation and Adaptation Layer
(SEAL)", and the Reference is the RFC-to-be currently known as
'draft-templin-intarea-seal'.
15. Security Considerations
Neighbor relationships between the ITE and ETE should be secured in
environments where authentication and/or confidentiality are a matter
of concern. Securing mechanisms such as Secure Neighbor Discovery
(SeND) [RFC3971] and IPsec [RFC4301] can be used for this purpose,
however the tunnel neighbor relationship is managed by the tunnel
protocols that ride over SEAL (as an encapsulation sublayer) rather
than by SEAL itself.
Security issues that apply to tunneling in general are discussed in
[RFC6169].
16. Related WorkSection 3.1.7 of [RFC2764] provides a high-level sketch for
supporting large tunnel MTUs via a tunnel-layer fragmentation and
reassembly capability to avoid IP layer fragmentation.
Section 3 of [RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU.
Section 4 of [RFC2460] specifies a method for inserting and
processing extension headers between the base IPv6 header and
transport layer protocol data. The SEAL header is inserted and
processed in exactly the same manner.
The concepts of path MTU determination through the report of
fragmentation and extending the IPv4 Identification field were first
proposed in deliberations of the TCP-IP mailing list and the Path MTU
Discovery Working Group (MTUDWG) during the late 1980's and early
1990's. An historical analysis of the evolution of these concepts,
as well as the development of the eventual PMTUD mechanism, appears
in [RFC5320].
17. Implementation Status
An early implementation of the first revision of SEAL [RFC5320] is
available at: http://isatap.com/seal.
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